[Fix] (DirectVulkan, MG_State): make a shader-written storage buffer readable on Magma

Reading a buffer a compute shader wrote gave zeros: the frontend shadow that MapBuffer
resolves against is only maintained by uploads, and Magma had no path back. Every
KHR-GL40.texture_gather case ends by dispatching a compute shader into an SSBO and
comparing the mapped result, so 66 of 75 failed on it.

Magma needs no readback: EnsureGpuResidentStorage - the same host-visible coherent
adoption the transform feedback capture already uses - makes the shadow BE the memory
the shader writes, so binding a buffer as a shader storage buffer now adopts it. What
coherence does not give is ordering: the writes are visible once they have happened,
and the CPU was reading before the dispatch had retired. The readback op therefore
submits the recorded work and waits.

That exposed a mistake in the frontend flag this rides on: MarkGpuWritten skipped
GPU-resident buffers, reasoning there was no shadow to refresh. True, but the wait is
still needed - "reconcile with the GPU write" is not always "copy it back", and which
of the two it is belongs to the backend. The flag now only says a write is outstanding;
DirectGLES's readback still skips its persistent-mapped buffers when copying.

KHR-GL40.texture_gather on Magma: 66 failures -> 19 (the rest are rectangle textures,
mipmap completeness and tessellation, all still to do). Espryt stays at 75/75.

MobileGL/MG_Backend/DirectVulkan/Renderer/VulkanRenderer.h

@@ -795,6 +795,14 @@ namespace MobileGL::MG_Backend::DirectVulkan {
                                            VkImageLayout finalLayout);
         Bool SubmitReadbackCommandsAndWait(FrameContext::FrameData& frame);
 
+    public:
+        // Submits whatever is recorded and waits for it. The CPU is about to read memory
+        // a shader wrote (a mapped shader storage buffer), and coherent host-visible
+        // storage only guarantees visibility once the work that produced it has retired.
+        Bool FinishPendingGpuWork();
+
+    private:
+
         void ShutdownSwapchain();
 
         // Static functions